In the mobile data communication system using CDMA or the like, channels for carrying out actual communications between a radio base station and a mobile radio terminal (mobile station) within a service area managed by the radio base station are classified according to their use as follows.
Namely, one is a common channel and the others are individual channels having a plurality of channels, where the former is a common channel to be used by a plurality of users together, which is used when a traffic from a user is sparse. The latter is a channel to be used exclusively by a user, which is set to be used when a communication traffic is dense. The data communications are carried out by appropriately switching between these two types of channels depending on a state of traffic from a user.
Also, in the mobile communication system using CDMA or the like, the judgement as to whether or not a call admission is possible is made by using a magnitude of an amount of interferences received at the radio base station, an information on a power level transmitted from the radio base station, and their respective prescribed thresholds. Moreover, in the mobile radio terminal, it is possible to judge a call admission autonomously at the mobile radio terminal by receiving a notification of information on an uplink interference amount and a downlink transmission power, information on thresholds, etc., from the radio base station through a broadcast channel.
However, in the mobile data communications which exchange data while switching a physical channel as described above, conventionally the judgement as to whether or not allocation is possible has not been carried out at a time of switching the physical channel, so that it has not been possible to prevent the quality degradation in the case where data traffics from a plurality of mobile stations are concentrated.
On the other hand, the call admission control that has conventionally been applied at the call level is also applicable to the data communications, but this method cannot suppress data transmission in the common channel, so that there has been a serious problem that no effect can be achieved by the straightforward application. Namely, when the admission is not possible as a result of the judgement, the mobile station will stay in the common channel and carry out data transmission in the common channel, but there has been a possibility of the quality degradation as the transmission radio signals sometimes give rise to interferences with respect to the other users who are communicating in the same band.